Seasonality of Upper-Ocean Vertical Eddy Buoyancy Flux in the Kuroshio Extension

Abstract

Abstract Oceanic mesoscale and submesoscale eddies produce a pronounced vertical buoyancy flux, playing an important role in ocean restratification. This study used a 1-km ocean simulation to investigate the seasonality of the vertical eddy buoyancy flux (VEBF) in the Kuroshio Extension as well as its underlying dynamics. The simulated VEBF in the upper 200 m over the Kuroshio Extension has a pronounced seasonal cycle. The winter VEBF peaks in the mixed layer, whereas the summer VEBF has a much smaller magnitude but a more complicated vertical structure with a narrow peak in the shallow mixed layer and a broader and stronger peak in the seasonal thermocline. The baroclinic instability (BCI), frontogenesis, and turbulent thermal wind (TTW) balance all contribute to the VEBF seasonal cycle. In winter, large surface heat loss and intense winds destroy stratification and enhance turbulent vertical mixing in the upper ocean. These phenomena intensify VEBF by promoting its components induced by the frontogenesis and TTW balance and by triggering mixed layer instability (MLI). In summer, strong stratification associated with suppressed turbulent vertical mixing weakens the contributions of the frontogenesis and TTW balance to VEBF and shifts the dominant BCI type from the MLI to the surface Charney- and Philips-like types with greatly reduced growth rate compared with that of MLI in winter. The shallow peak of the VEBF in summer is mainly attributed to the TTW balance, whereas the BCI and frontogenesis account primarily for its deep peak.